Scales for measuring earthquakes...

The Richter Scale is the best known scale for measuring the magnitude of earthquakes.
The magnitude value is proportional to the logarithm of the amplitude of the strongest
wave during an earthquake. A recording of 7, for example, indicates a disturbance with
ground motion 10 times as large as a recording of 6. The energy released by an earthquake
increases by a factor of 30 for every unit increase in the Richter scale. The table below
gives the frequency of earthquakes and the effects of the earthquakes based on this scale.

These effects are assuming a shallow earthquake in a populated area. Earthquakes of
large magnitude do not necessarily cause the most intense surface effects. The effect in a
given region depends to a large degree on local surface and subsurface geologic
conditions. An area of unstable ground (sand, clay, or other unconsolidated materials),
for example, is likely to experience much more noticeable effects than an area equally
distant from an earthquake's epicentre but underlain by firm ground such as granite.